In the past, substantially all such racks have been made from a cylindrical bar of steel having cut therein transverse teeth over about one quarter of the length extending from one end. Typically a flat is first machined on the bar to a depth somewhat less than half the radius of the bar, and the teeth have a depth of about half that of the "flat". The remaining depth of section through the bar beneath the teeth is thereby reduced to about two-thirds of its diameter, so reducing its resistance to bending to less than half. Such racks are made of medium carbon steel and have their teeth induction hardened to improve their resistance to wear.
As installed in automobile steering gears, such rack bars are subject to severe bending loads because of forces transmitted from the suspension through the tie rods to the overhung ends of the rack bar where it protrudes through the steering gear housing. Such bending loads reach the same maximum value on the right side of the vehicle as on the left, and hence, as the rack bar is designed to have adequate strength to resist this bending on the toothed end, it follows that it will have double the required strength in the cylindrical end. As the latter comprises about three-quarters of the length of the rack bar, it is evident that such a rack is far heavier than necessary and wastes material.
The above shortcomings of such racks as commonly made may be overcome by employing a rack whose cross section resembles ther capital letter "Y", but with the area between the upper limbs filled in and teeth cut therein.
Such a section, the use of which is described in U.K. Patent No. 1,525,760, resembles a girder and is strong for its weight in bending, its strength being diminished less by the cutting of the teeth than in the case of round rack bars.
In the following text the term "Y form rack" will be used to describe a steering rack of the type just described. However the comparison to the capital letter "Y" should not be held to imply that the underside of the lmbs are necessarily flat or that there need to be a stem or tail there between. The lower surfaces may, for instance, be made convex or concave in section and the surface between them may have a smaller discontinuity than implied by the term "Y form".
The lower side of the "Y" limbs of such racks must act as guide surfaces in the same manner as the cylindrical surface of conventional racks, and hence must be smooth and accurately related to the pitch line of the teeth opposite within a tolerance of 0.025 mms or less. Machining of these "Y" faces to such a finish and close relationship to the juxtaposed teeth is difficult by known machining methods.
In another recent development in steering rack bars, the regularly spaced teeth hitherto used are replaced by teeth of irregular form and pitch as described in U.K. Patent No. 1,356,172, providing a variable steering ratio. Such teeth offer considerable advantages in reducing the parking effort, but cannot readily be produced by any known method such as gear cutting, broaching or grinding.
As neither of the foregoing developments are readily amenable to conventional machining methods, most racks made to date incorporating them have had to be made by highly unsuitable forging methods, as will be described.
Such racks do have the advantage that, in forging, the grain of the steel is caused to flow around the contours of the teeth and transverse to their length, so enhancing the rack tooth fatigue strength, as is well known in the art of gear forging.
U.K. Patent No. 2,056,894 purports to show how steering racks, including those incorporating variable ratio, may utilize the above-described beneficial effect of forging, and also reinforce such effect by arranging that the grain of the bar of the material from which the rack is made also lies in a direction transverse of the teeth.
In actual fact the beneficial effects of the forging of gear teeth and the rolling of threads, on the fatigue strength, are well known and occur irrespective of the direction of the original grain direction. In any case the grain direction of the bar described in the patent would result, inevitably, from any process of forging transverse teeth in a rack bar such as described in U.S. Pat. No. 3,550,418 or referred to in one method described in U.K. Patent No. 2026908.
However, there are several serious defects of the method of manufacturing racks described in U.K. Patent No. 2,056,894. For example, it is immediately apparent that, if a round bar were squeezed between close fitting die halves, one having the obverse of the tooth forms therein, the steel would start squeezing out horizontally between the approaching die faces to form "ribs" just as soon as forming of the teeth commenced. Such "ribs" on the sides of the rack would prevent the die closing. Some squeezing of the ribs might occur but this would cease, if forging hot, when the steel was chilled by the die, or, if forging cold, when the steel work-hardened.
In either case the volume of metal so wasted in a die of the proportions shown in the specification of that patent could amount to one-fifth or more of the original blank, making it impossible that the blank volume precisely equals the volume of the finished rack. This wastage would be even greater if the rack teeth conformed to the conventionally used proportions referred to earlier. The round-tipped rack teeth shown are completey impractical for use in steering racks. The prior machining of a flat on the rack bar and the removal of the side ribs both call for additional operations and wastage of material.
Referring now to U.K. Patent Application No. 2088256, this overcomes some of the limitations of the patent referred to above. For example, FIG. 5 shows that provision is made for the formation of side "ribs" by suitable "gutters" on each side of the longitudial axis of the main cavities of the die as is well known in the art of forging. Furthermore, the process recognises the impracticability of finish forming such teeth to the required accuracy in such dies, either cold or warm, but rather specifies an initial operation be carried out at the conventional forging temperature (generally over 1000.degree. C.).
The patent specification recognises that distortion and scaling occur in forging at such temperatures, and therefore specifies that, after straightening and descaling operations, the rack bar in the toothed portion be cold coined to give the required precision. An additional disadvantage of this process, as described in that specification, is that, because of use of the high forging temperature, the core material of the racks becomes softened so that the forged blanks must be subsequently hardened and tempered in order to provide the necessary strength in the finished rack. Distortion inevitably occurs in such hardening and hence additional finishing operations are required in which the long end of the rack must be machined in exact alignment with the forged tooth end. The rib material which extends sideways from the root areas at each end of the tooth must be removed by trimming or cutting and hence the desirable grain direction wrapping around the root of the tooth is cut at the ends of the teeth where stresses tend to be highest and hence some of the fatigue strength attributable to forging is lost.
Some of the problems, for example, scaling, distortion and softening of the prior art just referred to may be overcome if forging occurs at lower temperatures, a process frequently referred to as warm forging, that is to say from about 550.degree. C. to 750.degree. C. A transition of the steel to the austenitic state, with accompanying dimensional changes, is thereby avoided. However, the steel is far less plastic at these temperatures than it would be at 1000.degree. C. and hence it is more difficult to completely fill the teeth.
An additional problem of the prior art is the lack of adequate restraint of the steel blank and the escape of material which therefore occurs during the final closing of the die; this prevents the use of such lower forming temperatures. To make possible the forming and filling of the teeth at lower temperatures the use of a die chamber that is as nearly as possible, is desirable.
This problem is recognised in the art of designing forging dies where very complete fill of the die cavity is required as occurs, for example, in the hot forging of brass or die casting of aluminium. In such dies reduced loss of material and an increase of forming pressure is realised by providing a chamber like die cavity in which the final forming pressure is developed by a piston-style die configuration which avoids any entrapment of material between the closing die faces; "nipping" or trapping of material between opposing die faces during the last instance of closure is avoided.
However the pressures and temperatures involved in the forging of medium carbon steels at the intermediate range temperatures specified exclude the possibility of using the piston style die configuration referred to in those other arts referred to above, making them inappropriate to solving the present problem.
The present invention provides a die suited to the forming of steering rack bars of the configuration described which fulfills all the above needs. Furthermore the invention makes possible a low cost method of making such steering racks involving fewer steps and wasting less material than the prior art. Finally, the invention makes possible the manufacture of a rack of improved design incapable of being manufactured by any other known technique.